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2009, 113, 9266–9269 Published on Web 07/28/2009
Nonlinear Photochemistry Squared: Quartic Light Power Dependence Realized in Photon Upconversion Tanya N. Singh-Rachford and Felix N. Castellano* Department of Chemistry and Center for Photochemical Sciences, Bowling Green State UniVersity, Bowling Green, Ohio 43403 ReceiVed: July 17, 2009
Simultaneous two-photon excitation of a solution mixture of [Ru(dmb)3]2+ and 9,10-diphenylanthracene (DPA) using 860 nm light pulses from a Ti:Sapphire laser resulted in triplet energy transfer followed by triplet-triplet annihilation (TTA), ultimately leading to upconverted DPA fluorescence from the sensitized DPA triplets. The photochemistry sequence was confirmed by the unprecedented quartic (x4) incident light power dependence exhibited by this process, which incidentally generated a record anti-Stokes shift of 1.38 eV for sensitized TTA. Introduction Sensitized triplet-triplet annihilation (TTA) photochemistry continues to emerge as a promising wavelength-shifting technology. This process generates higher-energy photons from the absorption of lower-energy light by an efficiently quenched triplet sensitizer through sequential, highly allowed one-photon absorptions, producing TTA from the energy-transfer products. In essence, the energy stored in two sensitized triplet molecules is combined to produce a higher-energy singlet state and a corresponding ground-state species. This strategy has proven exceedingly effective when metal-based sensitizers are combined in concert with aromatic triplet acceptors/annihilators.1,2 In terms of technological relevance, these low-power upconversion processes are readily visualized by the naked eye and have been successfully translated into a variety of solid polymeric host materials.1e-g,i Simultaneous two-photon excitation (TPE) processes are generally accomplished through the use of high peak power ultrafast lasers with wavelengths ranging from the visible to the near-IR. TPE of various chromophores has led to widespread fundamental scientific progress along with notable applications in chemistry, materials science, imaging, photolithography, and micro-to-nanofabrication.3-8 Inspired by the pursuit of new opportunities in photonics, we postulated whether the sensitized TTA process could be adapted to TPE technology. This would effectively replace the two sequential one-photon excitations typically associated with TTA with two independent simultaneous two-photon excitations, the latter exhibiting an unprecedented quartic (x4) incident light power dependence.9 As it is well established that RuII metal-to-ligand charge-transfer chromophores are susceptible to TPE10 and the [Ru(dmb)3]2+ (dmb ) 4,4′-dimethyl-2,2′-bipyridine) excited state successfully promotes sensitized TTA with 9,10-diphenylanthracene (DPA),1b this molecular donor-acceptor pair represent a rational departure point for the present prototypical study. * To whom correspondence should be addressed. Phone: (419) 372-7513. Fax: (419) 372-9809. E-mail:
[email protected].
10.1021/jp906782g CCC: $40.75
CHART 1: Chemical Structures of (a) [Ru(dmb)3]2+ and (b) DPA
Experimental Section General. [Ru(dmb)3](PF6)2 was synthesized according to the published procedure.11 9,10-Diphenylanthracene and spectroscopicgrade acetonitrile were purchased from Aldrich and used as received. Spectroscopic Measurements. The static absorption spectra were measured with a Cary 50 Bio UV-vis spectrophotometer from Varian. Single-wavelength emission intensity decays of [Ru(dmb)3]2+ in CH3CN were acquired with a N2-pumped dye laser (2-3 nm fwhm) from PTI (GL-3300 N2 laser, GL-301 dye laser) using an apparatus that has been previously described.12 Coumarin 510 was used to tune the unfocused pulsed excitation beam. Two-photon excitation measurements were accomplished with a Ti:Sapphire laser (Chameleon Ultra II, Coherent) tuned to afford 860 nm excitation at 80 MHz. The laser output was focused onto the sample cell (1 cm path length cuvette) with a planar convex lens of 65 mm focal length placed inside a time-correlated single-photon counting spectrometer (Edinburgh Instruments, LifeSpec II). The emission was collected using a microchannel plate photomultiplier tube (Hamamatsu R3809U-50) in a Peltier-cooled housing. One-photon excitation at 430 nm was accomplished by frequency doubling the 2009 American Chemical Society
Letters
J. Phys. Chem. A, Vol. 113, No. 33, 2009 9267
Figure 1. Normalized absorption spectra of [Ru(dmb)3]2+ and DPA in CH3CN. The blue and red arrows indicate the experimental positions used for selective one- and two-photon excitation (OPE and TPE) of [Ru(dmb)3]2+, respectively.
860 nm excitation output from the Ti:Sapphire laser using a second-harmonic generator (Angewandte Physik & Elektronik GmbH). The power of these laser beams was measured using a Molectron Fieldmate sensor and power meter (Coherent). The optical power of the laser beam at the sample was controlled by placing a series of neutral density filters in front of the sample. All samples were deoxygenated for at least 30 min with high purity argon prior to all measurements, and a positive argon pressure was maintained throughout the experiments. All spectral data were collected and processed separately in Origin 8.0. Stern-Volmer Quenching. The Stern-Volmer and bimolecular quenching constants were obtained according to the Stern-Volmer relation shown in eq 1
I0 /I ) 1 + KSV[Q]
(1)
where I0 and I represent the photoluminescence intensities in the absence and presence of the quencher, respectively, KSV is the Stern-Volmer constant (KSV ) kqτ0) and [Q] is the molar concentration of the DPA energy-transfer acceptor. In a typical experiment, 9.6 µM [Ru(dmb)3]2+ was deaerated with argon for at least 30 min, and the emission intensity was collected upon 860 and 430 nm excitation, measured as a function of DPA concentration. The determined Stern-Volmer (Ksv) and the bimolecular (kq) quenching constants obtained upon one- and two-photon excitation are each the average of two independent measurements at each excitation wavelength. Results and Discussion The chemical structures of the two chromophores pertinent to this investigation are shown in Chart 1, and Figure 1 displays the absorption spectra of the chromophores measured in CH3CN. The colored arrows indicate the respective wavelengths of the excitation light utilized. Ti:Sapphire laser output (Chameleon Ultra II, Coherent) at 860 nm was chosen as this wavelength provided the highest possible incident power while preserving selective TPE of the [Ru(dmb)3]2+ chromophore in the presence of DPA. Quenching of the [Ru(dmb)3]2+ photoluminescence with increasing DPA concentration was quantitatively measured following both onephoton excitation at 430 nm (10 mW @ 80 MHz) and TPE at 860 nm (1 W @ 80 MHz) in argon-degassed CH3CN (Figure 2). Stern-Volmer analyses of the data obtained from two independent sets of experiments (
